1. Field of the Invention
The present invention relates to a hydraulic shock absorber in which the damping force characteristics using a bulk modulus of a hydraulic fluid can be obtained more effectively when the moving speed of a piston rod in relation to a cylinder tube is slow as in the initial stage of compression action and extension action, and in a case in which the hydraulic shock absorber repeats the compression action and the extension action by external first and second input forces.
2. Description of the Related Art
A conventional type of hydraulic shock absorber is shown in JP-A-Hei 10-331898. According to this document, the hydraulic shock absorber is slidably inserted in a cylinder tube in the axial direction, and the hydraulic shock absorber includes first and second pistons for dividing the inside of the cylinder tube into a first chamber, an intermediate chamber, and a second chamber from one end to another end in this order; and a piston rod in which one end is connected to the first and the second pistons, and another end protrudes out of the cylinder tube from another end of the cylinder tube.
The shock absorber also includes a first damping force generating device for absorbing a first input force by oil flowing from the first chamber through the first piston to the intermediate chamber, when the first input force is externally applied in order to insert the piston rod deeper into the cylinder tube; and a second damping force generating device for absorbing the first input force by oil flowing from the intermediate chamber through the second piston to the second chamber when the first input force is applied.
The first and second damping force generating devices work concurrently to generate the damping force when the shock absorber is supplied with the external first input force and performs an action to insert the first and the second pistons and piston rods into the cylinder tube, that is, when the shock absorber is in compression.
The shock absorber in the above case has the following damping force characteristics. An increase gradient of the damping force is large when the compression action of the shock absorber is at the initial stage and when the moving speed of the piston rod in relation to the cylinder tube is slow. After this, when the moving speed increases, the damping force becomes higher. However, the increase gradient decreases gradually (see, for example, paragraphs in JP-A-Hei 10-331898).
When a vehicle is driving, at the initial stage of the compression action in which the shock absorber is supplied with the first input force from the driving road surface through the vehicle wheel side, a first orifice and bypass passage that are part of the first and second damping force generating devices work together concurrently and generate the damping force. At this time, the second chamber expands and its hydraulic pressure decreases to lower than that of the intermediate chamber.
In general, a hydraulic fluid used in the shock absorber is a compressible fluid with a characteristic performing a volume change by pressurization or decompression (hereinafter, this is simply referred to as “bulk modulus”). A capacity of the second chamber alone is small, so if compared with an imaginary chamber that has the same pressure value and a larger capacity than the second chamber, the volume change amount of the oil in the second chamber according to the “bulk modulus” is smaller (harder) than that of the imaginary chamber.
Therefore, even at the initial stage of the compression action of the shock absorber, the hydraulic pressure in the second chamber rapidly decreases according to the entering action of the second piston. Following this, the oil flows immediately from the intermediate chamber through the second damping force generating device to the second chamber in order to generate the damping force. In other words, the response of the damping force generation in relation to a time that the external force is applied, which is the first input force applied to the shock absorber, becomes high (hereinafter this is simply referred to as the “response”).
Thus, at the initial stage of the compression action of the shock absorber, the driver tends to be given a hard impression, because the response is high. This is not preferable since it fails to improve driving comfort of the vehicle.
On the other hand, when the shock absorber is supplied with an external second input force, then the first and the second pistons, and the piston rods perform an action to extend from the cylinder tube. That is, when the shock absorber is performing such an extension action, the second chamber performs compression at the initial stage, and its hydraulic pressure becomes higher than that of the intermediate chamber.
Here, the capacity of the second chamber alone is small, so the volume change amount by the “bulk modulus” of the oil in this second chamber is small (hard). Therefore, the hydraulic pressure in the second chamber rapidly increases according to the extension action of the second piston, even at the initial stage of the extension action of the shock absorber. Following this, the oil immediately tends to flow from the second chamber through the second damping force generating device to the intermediate chamber. That is, the response becomes higher.
Therefore, a driver tends to be given a hard impression from the vehicle even at the initial stage of the extension action of the shock absorber, and the same problem as mentioned above may occur.